Removed the use of the environ variable and instead use the environment array

Removed the use of the environ variable and instead use the environment array
passed into main().  This may (or may not) be more portable, but it looks
nicer.
Added functions to copy the environment array and modify the copy, thus
preserving the environment.
Switched from using system() to using ExecWait().
The code now removes the assembly file generated when using the -native option.
Fixed the remove_env() function: The previous revision truncated the
environment variable list.  This version just zaps the variable as intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8579 91177308-0d34-0410-b5e6-96231b3b80d8